In gas chromatography
a small sample of a mixture
of compoundsis vapourised
(turned into gas).
A carrier gas takes the sample through the column.
The carrier gas (often nitrogen)
must
be inert and notreact with the sample.

The column is packed
with a solidmaterial
whichslows the sample down.
Differentcompounds
travel
through the column at differentspeeds and leave
the other end of the column at differenttimes. The
amount of time
a particular compound takes to pass
through
the instrument is called its retention time.
The retention time can help identify
the compound.

A recorder draws a graph
called a gas chromatogram
which shows a peak for each
compound.
The number of peaks
show the number of compoundspresent in the sample and the position of the peaksshow the retention time. See the example
shown below.

In the gas
chromatogram above, there are fivepeaks
showing that there are five compounds present. Theheight of the peak
shows how much of the compound
there is in the mixture. A more accurate way
of identifying the differentcompounds
which leave thecolumn
can be made by using a mass
spectrometer.